A huge competitive market exists for disk drives that store massive amounts of data in computers. This competition requires disk drive manufacturers to provide ever-increasing data storage capacity and higher performance in their products. One way to increase capacity is to provide more data tracks on a given disk surface, which generally requires that the tracks be more densely packed since the form factor of the disk is limited by compatibility standards. In current disk drives, track density or pitch of 10,000 tracks per inch is becoming available.
As tracks are placed closer together, the problem of maintaining a read/write transducer in position over the track becomes more difficult. The industry presently prefers a rotary type actuator, which employs a voice coil motor to pivotally swing an array of vertically stacked arms carrying read/write transducers over tracks disposed on surfaces of a corresponding stack of disks. A sampled servo system reads servo wedges interspersed at equal intervals around the data tracks to maintain the position of a transducer over a track. At some point in the march toward higher track densities, these conventional actuators and servo systems may be unable to provide the bandwidth and precision control required to keep the head positioned over a target track without encroaching adjacent tracks.
Dual stage actuators have been proposed as a solution to the bandwidth and precision control problem. The dual stage actuator generally provides two pivot points and two motors with a second stage motor and pivot point being smaller and positioned closer to the transducer, therefore providing for higher control bandwidth and precision. Typically the span or range of motion of the second stage actuator is limited to a few tracks or even to the width of a single track or less.
Dual stage actuators may be broadly characterized as milli-actuators or micro-actuators. In a milli-actuator design, the second stage actuator moves a suspension arm that suspends the read/write head. The milli-actuator can adjust the radial offset of the read/write head by moving the suspension arm, and thus the read/write head, in relation to the first stage pivot point.
In the micro-actuator design, the second stage, mounted on the suspension arm, moves the read/write head directly. The micro-actuator can adjust the position of the read/write head by moving the read/write head in relation to the suspension.
The performance of disk drives employing dual stage actuators, as in conventional disk drives, is and will be impacted by the time required to position a transducer over a target data track to perform a read or write operation, characterized as access time. One major component of access time is the time required to move or swing the actuator from a present track to a target track, known as seek time. Another significant component of access time is the time required to perform a head switch, which entails selecting a target transducer head on a different one of the vertically stacked arms of the actuator and establishing the precise position of that head over the present track. The head switch occurs frequently during read or write operations of sequential data where data is conventionally recorded on a stack of respective tracks at the same radial location on each disk surface, known as a cylinder. This recording technique has been historically employed to minimize the delay in continuing sequential data transfer because a head switch can be accomplished in less time than a seek.
Although the stacked arms and transducer heads in a disk drive are closely aligned, there are sufficient variations between the disks, arms and heads to require that the target head be re-positioned when a head switch is performed, even though the previous head was perfectly aligned over a corresponding track. This is especially true with more densely spaced tracks and the problem may even be exacerbated with dual stage actuators. The instantaneous difference in position between heads on the stacked arms owing to these variations may be termed radial offset. As a consequence, there is a continuing need to improve head switch times and provide higher performance disk drives.